The cannibalism trial of a New York police officer is set to become darker yet if prosecutors succeed in showing jurors pictures of dead and dismembered people as they wrap up their case.

The dark twists at the cannibalism trial of a New York police officer will continue if prosecutors succeed in showing jurors pictures of dead and dismembered people as they wrap up their case. Defense lawyers were opposing the presentation on Friday of as many as 34 ghastly exhibits of images the government says it took from officer Gilberto Valle's computer.

US district judge Paul Gardephe said he'll decide when the time comes whether jurors will see pictures of dead and mutilated women that defense lawyers say may have been saved on the 28-year-old officer's computer automatically without him ever seeing them when he went on certain web sites. The government says the exhibits include a picture of a dead body whose feet were not attached that Valle's wife testified she saw when she went to one of his favorite internet sites as she discovered why he stayed up late at night on the internet.

The photographs were discussed out of the presence of jurors. They did hear an FBI agent testify that Valle's New York Police Department supervisor was among women the officer considered a potential target for a kidnap and torture.

Developments on Thursday were not limited to the courtroom. It was reported that a man considered to be one of Valle's co-conspirators, identified in the US case by the online name Moody Blues, had been detained but released in a separate case in Britain.

In Manhattan federal court, FBI agent Corey Walsh told jurors that a search of Valle's computer turned up a file with multiple photos of a woman the agent identified as an NYPD officer who supervised Valle. The government had previously introduced a transcript of a February 2012 email exchange in which Valle offered a co-conspirator a menu of women he could abduct for rape and torture.

"The second girl listed is a cop - Evelyn, 33 years old," Valle wrote.

"No I want a reg girl," the man responded.

The revelation came as the defense sought to discredit allegations that Valle conspired with internet friends to kidnap, kill and eat women, pressing Walsh on why some communications were deemed proof of a crime while others were deemed fantasies.

Defense attorney Robert Baum directed Walsh to obvious falsehoods in communications that the government has used as evidence Valle was a threat. In one, Moody Blues insisted he and Valle would need a secluded place to cook a woman alive.

"I have a place on the mountains," Valle wrote. "Nobody's around for three quarters of a mile."

Asked if that was true, Walsh testified that authorities "are not aware of a place he had in the mountains".

Valle has been held without bail since October, when he was arrested on charges of conspiring to kidnap women in a cannibalism plot born on the internet. Throughout the trial, which began on Monday, Valle's lawyers have attacked government evidence as nothing more than the reflection of a man engaging in extreme sexual fantasies with like-minded people around the world. The government has conceded that Valle never met the purported internet co-conspirators and no women were harmed.

Even the judge Thursday as he issued a legal ruling said "there is almost no evidence of action by Valle beyond computer-based activity".

Jurors have heard from Valle's estranged wife and from former classmates and other women who testified they knew Valle on a casual basis and never considered him dangerous. Their testimony was followed by evidence that all of them were the subjects of emails and chats describing how they could be snatched away and eaten.

One of the women, special education teacher Alisa Friscia, appeared angry and agitated as she testified on Thursday that she had worked with Valle's wife but wasn't close to her.

As for Valle, she snapped, "No, he's never been a friend!"

She was followed to the stand by police officer Robin Martinez, a police academy instructor, who said there were severe penalties for unauthorised searches of the national police database that Valle was accused of abusing to collect information about targetted women.

"You are going to face serious trouble, including being prosecuted, being arrested, being fired," she said.

Prosecutors allege Valle's cyber co-conspirator Moody Blues told the officer he had killed and eaten at least two women. The New York Post reported on Thursday that two men had been arrested in a child porn case in Britain and that one was Moody Blues.

When asked on Thursday whether there had been any arrests related to the Valle prosecution in New York, local police in the southern England county of Kent said they had briefly detained two men from the area around Canterbury, the famous cathedral city about 96 km southeast of London.

Kent police identified the men only as a 57-year-old and a 30-year-old, saying that they had been arrested a week ago on suspicion of conspiracy, child grooming and possession of child abuse images.

No charges had been filed, and police said the men have since been released on bail.

The statement said Kent police had "been in contact with US law enforcement agencies in relation to this investigation" but did not go into any further detail. An FBI spokesman in New York declined to comment.